The contract also includes the supply of other assets including hang-offs, connectors and a cable protection system.
JDR said it would manufacture and "pre-commission" the cabling system at its facility in Hartlepool, northeast England.
E.on made its final investment decision on the project in May, at the same time as the Green Investment Bank made a £236 million investment (€325 million) in the project. GIB acquired a 19% stake in the project.
Earlier this month, electrical systems specialist ABB won the contract to supply the project's substations.
Rampion will be the first Round Three project to begin construction in the UK. It is expected online in 2018. The project was originally planned to be 700MW, but was cut in size in November.